The ability of alkaline electrolytes to wet a metal surface is one of the major causes of leakage in alkaline galvanic cells. The alkaline electrolyte, by wetting the surface of the metal, can actually "creep" through a sealed metal interface. For this reason, elaborate precautions have been made to prevent the electrolyte in an alkaline cell from contacting the interface between a seal and certain metal parts of a seal, such as the metal container and the metal cover of the cell. Typical cell systems where this problem is encountered include silver oxide-zinc cells, nickel-cadmium cells, air-depolarized cells and alkaline manganese dioxide cells. In the prior art it has been a conventional practice to incorporate insulating gaskets between the cell container and cover so as to provide a seal for the cell. Generally, the gasket must be made of a material inert to the electrolyte contained in the cell and the cell environment. In addition, it had to be sufficiently flexible and resistant to cold flow under pressure of the seal and maintain these characteristics so as to insure a proper seal during long periods of storage. Material, such as nylon, polypropylene, ethylene-tetra-fluroethylene copolymer (Tefzel) and high density polyethylene have been found to be suitable as gasket materials for most applications.
However, due to the propensity of alkaline electrolytes, such as aqueous potassium hydroxide and sodium hydroxide solutions, to wet metal surfaces, the use of gaskets, such as nylon, have not completely solved the leakage problem. Unfortunately, electrolyte leakage not only depletes the electrolyte solution from the cell but can also cause a corrosive deposit at the cover-container interface that not only affects the cell's appearance but could also cause damage to the device in which the cell is used. In addition, if leakage occurs while the cell is being transported and stored at a retailer's establishment, then the cell becomes unmarketable, resulting in a total loss.
U.S. Pat. No. 3,922,178 discloses a sealing means for alkaline batteries which comprises the use of a polyamide, specifically a fatty polyamide, as a sealant or protective coating to prevent alkaline electrolyte from wetting surfaces within the cell.
U.S. Pat. No. 3,713,896 discloses an insulating seal for alkaline cells which consists of a layer of cured epoxy-polyamide resin located and compressed between a metallic container and a metallic cover.
It is an object of the present invention to provide an electrochemical cell with an improved sealing means to prevent electrolyte leakage from the cell.
It is another object of the present invention to provide a secondary seal for electrochemical cells which comprises a nonconductive gasket coated with a fatty polyamide (primary sealant) disposed and squeezed between the interface of the cover and container of the cell and wherein a bead of a polyamide adhesive (secondary sealant) is disposed on the top edge of the gasket and extends over to contact and at least partially cover the edge of the container and the peripheral edge of the cover and serves as a physical barrier against electrolyte creepage and/or leakage at the interfaces of the container-gasket and cover-gasket.
The invention also relates to a method of sealing electrochemical cells whereby a polyamide adhesive is disposed on the top of the gasket between the cover and the container of the cell and then the container is crimped or curled over the gasket and the cover whereupon the polyamide adhesive forms a barrier bead secured to the top edge of the gasket and extending over to contact the peripheral edge of the cover and the edge of the container.
The foregoing and additional objects will become more fully apparent with the following description.